Silicon nitride films are dielectric films that have been used as insulating and passivating or protective films for integrated circuits. Such films include silicon nitride (Si.sub.3 N.sub.4) and silicon oxynitride (SiON) films. Protective films can be formed by chemical vapor deposition (hereinafter CVD) at high temperatures of about 600.degree.-700.degree. C. by pyrolysis of silane for example. However, when metal contacts are made from aluminum, aluminum alloys or other low melting metal, processing temperatures after formation of the contacts are limited to about 450.degree. C. to avoid damage to the metal contacts. Plasma processing is compatible with these low temperature requirements. Thus plasma enhanced chemical vapor deposition (PECVD) has also been tried for depositing protective silicon nitride films at low temperatures that will not adversely affect metal contacts or otherwise damage the integrated circuit. Also, when PECVD is used to deposit silicon nitride films for use as intermetal dielectrics, the conformality of the films over stepped topography is good. The usual starting material for silicon nitride deposition is silane (SiH.sub.4) but organosilane starting materials, such as hexamethyldisilazane (HMDS), are also used.
One of the problems with low temperature deposition of silicon nitride films from silane is that large amounts of hydrogen are incorporated into the films as Si--H and N--H bonds. The presence of large amounts of hydrogen contributes undesirably to a high wet etch rate for these films, so that when a subsequent etch step is required, as for patterning the silicon nitride film, etch rates are up to 100 times faster than silicon nitride films made by high temperature processing. Thus silicon nitride films deposited by PECVD cannot be used as masking layers.
Silicon nitride and silicon oxynitride films deposited by PECVD from organosilanes do not contain large amounts of hydrogen, and thus these films have lower wet etch rates. However, films from organosilanes do contain high amounts of carbon, and the conformality of these films with respect to deposition on stepped topography is poor. In addition, at low deposition temperatures, polymeric products also tend to form from organosilanes, which precludes their use as final protective films for integrated circuits.
Thus a method of forming silicon nitride and silicon oxynitride films at low temperature, but without formation of polymeric residues, to produce a film having a low wet etch rate, low carbon content and with good conformality over stepped topography would be highly desirable.